Why do I get worse gas mileage from the second half tank of gas than I do from the first?

Dear Car Talk

Dear Car Talk | Aug 01, 1999

Dear Tom and Ray:

While I know I should be worrying about the tension in Kosovo, I am instead trying to figure out the answer to a dumb question. Why do I get many more miles out of
the first half of my tank of gas than out of the second half? For example, my car gets about 200 miles out of the first half-tank of gas. You would think, then, that I
would get 400 miles from a full tank. But I don't. I get only another 100 miles or so as the needle goes from half-full to empty. This has been true of other cars I've
owned, too. Why is this? -- Frank

RAY: You're absolutely right, Frank. It IS true of every car I've ever owned, too.

TOM: I can't say whether or not my '63 Dodge Dart ever did this. I never filled it up all the way because I didn't want to invest more money in the car than it's worth.

RAY: Here's what's going on: Even after your gauge reads "Full," the tank itself is not necessarily completely filled up. You can always cram in an extra gallon or two.
Sometimes more. So you've got gas at the top end of the tank that the gauge doesn't even see.

TOM: So right away, you're starting with a couple of "bonus" gallons at the top. Then, you get "cheated" out of a couple of gallons at the bottom end of the tank, because
even when the gauge reads "Empty," there's still more gas in there. And the amount varies tremendously from car to car.

RAY: Here's how it may work using hypothetical numbers. Let's say the actual capacity of your gas tank is 20 gallons. The gauge may only show you what's between the
18 gallon mark (Full) and the 2 gallon mark (Empty). The gauge would read "half-full" when you had 9 gallons left.

TOM: So after you've "filled it up," you have to burn off 2 gallons of fuel before the needle even starts to move down from the "full" mark. That means you actually DO
go farther on the first half of your tank-full. And by the time you've gotten down to the halfway point (from 20 gallons down to nine), you've had the benefit of 11
gallons of gas.

RAY: Whereas, in the second half, you only get 7 gallons before the gauge reads empty (from nine down to two). And that's only if you run it all the way down to
empty, which most people don't.

TOM: Why do most manufacturers make their gauges like this? It's not that they can't make them more accurate. It's just that they don't want you to overfill the tank or
run out of gas.

RAY: Overfilling the tank can ruin the charcoal canister that traps fuel vapors. It's part of the emissions system and is therefore the manufacturer's responsibility for the
first 100,000 miles.

TOM: And they don't want you to run out of gas for two reasons. One is that, as we all know, they are great humanitarians and don't want you to have to walk home on
a cold, dark, rainy night. But more importantly, the car's fuel pump lives in the gas tank and is cooled by the gasoline. And they'd feel just terrible if you ran out of gas
and burned out the fuel pump -- especially while the car was covered by THEIR warranty.

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